Load Rejection — Mechanical
Load rejection is the sudden removal of the load from a running engine. At that moment the fuel is still set for the previous load, so the speed climbs rapidly; it overshoots rated speed until the governor cuts fuel back. This transient rise is the most typical situation in which overspeed protection can trip.
The sequence of events is this: a breaker opens or a pump unloads, the resistance disappears, and the engine accelerates. During the governor's response time the speed peaks, then comes back down and settles at idle speed.
- The peak value depends on the engine's inertia, the speed of the governor and how abruptly the load was removed.
- Overspeed protection typically trips at 10–20 percent above rated speed; in a poorly adjusted system load rejection can exceed this threshold.
In the field this corresponds to situations such as cavitation in the pump line or a valve opening suddenly. The critical warning here is this: the GS740 and GS740-3P are not an overspeed protection. The upper limit defined in these kits only limits the mechanical stroke of the actuator; the speed overshoot on load rejection is prevented by the engine's own governor and by an independent overspeed protection if one is fitted.
The context in which this term is used: Electronic speed and throttle control on a diesel engine
Do not confuse: what is described here is the speed overshoot caused by removal of the mechanical load on the engine. For the voltage transient that appears on the supply line when the battery is disconnected, see the entry Load Dump — Electrical.